What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Carrier Wave, but no Modulation

T

tinman

Well Known Member
I have a DRE 244 and Garmin SL40. I put power to them for the first time this weekend and have a strange bug to work out. I am getting good intercom function between the two headsets. When I key the mic to xmit, it get a carrier wave as verified by a scanner radio. I am not getting any voice transmission over the radio. I did manage to get one successful voice transmission, but that was it...
Any suggestions on where I should start my electron hunt?
Thanks
Don
 
Mic audio line

Check mic audio connection between intercom and SL40. If okay, try to find someone else with an SL40, swap out, and see if same problem. Otherwise, and not being familiar with your intercom, something is set/wired wrong with it.
 
Mis-wire?

Yes... sounds like the Mic Key is connected and working, but the Mic Audio In (in the the SL-40, that is) is not. Probably the Mic Audio wire from the intercom to the SL-40 is mis-wired or a faulty pin in a connector.

Also check the correct grounds are made to the SL-40 and the intercom....

gil in Tucson
 
I didn't consider a faulty pin...since it worked for one transmission, the wiring must be correct. I will start seaching for bad connection.
 
Hi Don:

I have the exact same setup as you, DRE 244E + SL-40. Are you using the DRE harness or did you build your own?

I had no problems wiring mine (except for the fact that DRE uses TINY wire in their harnesses which is a pain to deal with).

Is your SL-40 new? If so, go into the settings (by holding down the mon key) and make sure that the built-in intercom functionality is disabled.

Also, double-check the solder job at the jacks. If you plug in two headsets does the intercom function work fine? Do you get side-tone int he headsets when you talk?

Have you tried pressing the pilot's PTT while talking through the pax mic and vice versa? Maybe you have the pilot and pax ptt x-mit wires reversed?

If that doesn't work, send me an e-mail and we can work through your problem.

- Jamie
 
Thanks Jamie
The intercom is using the DRE-supplied harness and the SL-40 is also using a pre-made harness. I checked every permutation that I could think of as far as mic/ ptt cross-ups and so forth. I will verify that I have the SL-40's internal intercom function disabled, but I think that I do. The big mystery is how it managed to perform once correctly, then would not transmit voice again... I will do some more digging and will report back with any findings.
Don
 
One more thing...
When in pilot isolate mode, I lose the sidetone in the pilot's headset while transmitting...
Don
 
Hmm...I just checked the SL-40 install manual. The intercom select isn't in the menu...you enable the intercom by grounding one of the pins. Make sure that pin 12 on the SL-40 isn't grounded.

In the menu I would check the "transmit mic" setting. Mine is set to 1. When I get home this afternoon I can go through all of my settings and pins and let you know exactly what I did.

I have no explanation for it working once. I will tell you that I had one bad crimp on one of those tiny wires that actually broke the wire in the garage! Yikes! I redid the wiring out of an abundance of caution and made sure to do a very thorough job of vibration isolation. I would not be surprised to see that the audio line from your 244e to the SL-40 is broken.

Try this: pull the fuse/breaker on the intercom and try to transmit. If wired properly the 244e should failsafe the pilot's headset to your COM. If that works, you know that something is most likely wrong with the intercom. If it doesn't work, I would suspect your wiring or the SL-40.
 
Jamie,
Great minds think alike :)
Yesterday, I pulled the fuse on the intercom to see if the failsafe would work. I still got a carrier wave with no modulation. I think that we are pretty close to figuring this out. I will be in a world of hurt if I have to pull the connector off of the back of the sl40, though...there is no room to work there and I will have to pull out the entire radio stack to do this...stinking electrons!
 
Pins

If the connector pins are of the type that are inserted from the back into a shell, then there is a chance that they did not "snap in". When the connector is then mated, the pin can "push back" and become intermittent.

A visual inspection might show one pin not in the same depth as all of the others.... an easy check.... :)

gil in Tucson
 
+1 for Pins

Agree with Az-gila. I have an ICOM A200 with a Flightcom 403, and had the same symptom on initial test. After much testing, pulling of hair, swearing, consumption of beer, evaluation, etc. ..... it turned out that the mic audio pin in the connector on the back of the ICOM's rack was not quite fully inserted and snapped into place. It looked fine, visually, but was just shy of contacting the PCB on the back end of the radio when the radio was slid in. A slight tweak, and the radio was transmitting just fine.
 
FYI

Might not help with the current problem, but in general...
When you transmit on your radio you are sucking down a lot of power compared to rec. Check you battery to see if it is providing enough power.

When I started working on my panel a low battery turned out to be responsible for some strange noise and even stranger operating behavior.

Kent
 
I wanted to offer closure on this topic. After spending countless hours tracing the wiring and scratching my head, I called Stark Avionics where the radio was purchased. Tony at Stark said that I had done my part and arranged for me to return the unit to Garmin for a bench test. I shipped it off and am relieved to find out that the problem wasn't caused by my setup. There was a fault with the Tx circuit which was repaired by Garmin. My thanks go out to Stark for their willingness to stick their necks out for me and to arrange the test at Garmin. I have learned quite a bit about diagnostics lately which can be a good thing...
 
You must log in or register to reply here.
Back
Top